rust: alloc: simplify with NonNull::add() now that it is stable

Currently, we need to go through raw pointers and then re-create the
`NonNull` from the result of offsetting the raw pointer.

`feature(non_null_convenience)` [1] has been stabilized in Rust
1.80.0 [2], which is older than our new minimum Rust version
(Rust 1.85.0).

Thus, now that we bump the Rust minimum version, simplify using
`NonNull::add()` and clean the TODO note.

Link: rust-lang/rust#117691 [1]
Link: rust-lang/rust#124498 [2]
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260405235309.418950-15-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/kernel/alloc/allocator/iter.rs

@@ -42,15 +42,9 @@ impl<'a> Iterator for VmallocPageIter<'a> {
             return None;
         }
 
-        // TODO: Use `NonNull::add()` instead, once the minimum supported compiler version is
-        // bumped to 1.80 or later.
-        //
         // SAFETY: `offset` is in the interval `[0, (self.page_count() - 1) * page::PAGE_SIZE]`,
         // hence the resulting pointer is guaranteed to be within the same allocation.
-        let ptr = unsafe { self.buf.as_ptr().add(offset) };
-
-        // SAFETY: `ptr` is guaranteed to be non-null given that it is derived from `self.buf`.
-        let ptr = unsafe { NonNull::new_unchecked(ptr) };
+        let ptr = unsafe { self.buf.add(offset) };
 
         // SAFETY:
         // - `ptr` is a valid pointer to a `Vmalloc` allocation.